[dcache-ci]

Suggested documentation update for 158529f ("Merge branch 's3-for-master'"). AI-generated — please review for accuracy before merging.

Why this commit needs docs. The merge introduces the new S3 door (the whole of modules/dcache-s3 and modules/s3proxy-vendored), a new user-facing service that site administrators must set up. It also updates config.md and index.md to link a new chapter config-s3.md from the book's tables of contents, and adds the s3 principal to the principal table in config-gplazma.md — but docs/TheBook/src/main/markdown/config-s3.md is not present in the commit, so the book is built with dangling links to the new service.

What this PR does. Adds the missing config-s3.md, covering:

• running the door and choosing HTTP or HTTPS;
• the access key file (s3.authn.keystore.file) and mapping access keys to dCache users with a map plugin (s3:<access-key>) and a session plugin;
• how buckets and object keys map onto the namespace via s3.root, the keys dCache cannot hold, reserved names and the user's root directory;
• permissions, ETags and metadata stored as s3.* extended attributes;
• writing objects and multipart uploads, and the cleanup of old uploads (s3 uploads ls|rm|sweep);
• files that are on tape only (storage class GLACIER) and RestoreObject;
• client examples for the AWS CLI, rclone and s3cmd;
• the data path, scaling, virtual-host style requests, logging and admin commands;
• limitations.

Please note while reviewing. The chapter is written to match the tables of contents already added by this commit, so its section headings are fixed by those anchors (#introduction, #setting-up, #buckets-and-objects, #writing-objects, #files-on-tape, #clients, #operation, #limitations).

Also: while preparing this, master has since gained c485d1b ("[skip-ci] docs: add S3 door chapter to the dCache Book"), which adds an equivalent chapter for the same feature. This PR was opened from the commit under evaluation (158529f), where the file is absent; if c485d1b already covers it, this PR should simply be closed in favour of that commit. Any details worth keeping from this draft can be merged into the existing chapter.